1) Field of the in Inventions
The present invention relates to a gas spring, in particular for moulds, equipped with improved sealing means.
2) Background Art
As it is known, a gas spring is a device which, unlike conventional springs, uses a pressurized gas contained in a cylinder and compressed in a variable manner by a piston rod to exert a return force. Gas springs are used in numerous applications, in particular in the automotive, medical, furniture, aerospace and moulding sectors.
In this latter case, the gas springs for moulds are normally charged with nitrogen N2 at pressures of over 100 bars and constituted by a cylinder adapted to contain said gas therein and inside which the rod can slide: to allow the gas to be stored and compressed inside the cylinder one or more seal elements or gaskets are normally interposed between said cylinder and the rod, and these seal elements generally comprise at least one guide bushing and at least one rod seal. Therefore, starting from an idle position of the gas spring, to take the rod to the end of its travel it is necessary to apply a force thereon, this force naturally being greater than the initial force of the gas spring: the return movement of the rod, when the action of the applied force ceases, is instead automatic and caused by the pressure exerted by the gas on the rod inside the cylinder.
In prior art gas springs the guide bushings are produced with nylon, Teflon or other polymers, while the rod seals are produced with TPU (thermoplastic polyurethane), PU (polyurethane) or other polymers.
Typically, prior art gas springs have a useful life, intended as number of cycles that the rod can perform, beyond which gas starts to leak causing progressive loss of performance of the springs until reaching complete exhaustion. Normally, gas leakage occurs when the rod seal no longer adequately seals the surface of the rod due to wear caused by sliding on the surface of the rod and by the extrusion present between the surface of the rod and the surface of the guide bushing or of the cylinder.
In order to improve the resistance to wear and extrusion, prior art suggests the use of different fillers to mix with TPU, nylon or Teflon to produce rod seals and guide bushings. These fillers are usually constituted by lubricants, usually composed of PTFE (polytetrafluoroethylene), graphite or silicones, and/or reinforcing agents, usually composed of organic and inorganic fibres.
Prior art also suggests filling TPU with barite (BaSO4) for specific and limited applications such as those below:                in the medical field BaSO4 fillers are used to allow a component produced with TPU (such as a catheter), which would otherwise be invisible, to be viewed using X-rays;        given the high specific weight of BaSO4, it is added to the TPU to increase its weight for commercial purposes;        BaSO4 is used in the mechanical industry for grinding wheels as at low temperatures it promotes the formation of Fe3O4 on the surface of ground work pieces according to the reaction BaSO4+3Fe→Fe3O4+BaS, the Fe3O4 having a lubricating action.        